It is known to vary the length of an optical path in a transparent medium and thereby to change the phase of a light beam being transmitted along the path by using a device known as a phase modulator. Such a device may be an integrated electrical device such as a PIN diode that comprises heavily doped regions (n-doped and p-doped respectively) adjacent to the optical path. By passing a current through the PIN diode, carriers are injected into the transparent medium forming the adjacent portion of the optical path so that the refractive index of that portion of the optical path is changed. This change of refractive index effectively results in a change in the length of the optical path. This change in the length of the optical path results in a change in the phase of a light beam being transmitted along the optical path.
Integrated passive or active silicon-on-insulator (SOI) waveguides forming optical paths have very broad applications. Active integrated optical elements coupled to the waveguides may be based on phase modulators, such as the PIN diode modulators described above. Many integrated optical devices such as interferometers, switches and amplitude attenuators can be made from this integrated phase modulator structure. When a phase modulator is coupled to one of the optical paths of an optical system as above having two optical paths the difference between the path lengths of the two optical paths can be varied. The nature of the interference between the two light beams will therefore be varied.
Most phase modulators, such as the PIN diode phase modulator described above, require the use of a current driver. Due to the nature of the current response in most phase modulators the change in the optical path length and the phase change is a non-linear function of the driving current. In addition, due to the scattering of the light by the carriers injected into the material of the optical path, a light beam transmitted through the portion of an optical path which is coupled to a phase modulator including a PIN diode also undergoes amplitude modulation.
Another type of phase modulator is a thermal phase modulator. In this type of modulator a voltage is applied to heating or cooling means change the temperature of the material so that the phase of a light beam being transmitted along the path is varied. As for the above-described PIN diode type of phase modulator, the change in phase is a non-linear function of the voltage applied.
Integrated optical systems that include phase modulators are, for example, used in sensor applications, where the sensor system depends on the phase modulator to demodulate or process the signal. Any non-linearity of the phase modulator is thus reproduced in the sensor system output. Therefore, the non-linearity of prior art phase modulators directly affects the accuracy of the sensor system. Complicated linearization circuits have therefore been necessary in prior art systems to compensate for this deficiency.
Because of the nature of some phase modulators, such as the PIN diode phase modulator described above, the driving current always flows through the modulator in only one direction. Therefore, these phase modulators can only be driven in one direction and a standard push pull method of operation in which current can be made to flow through the phase modulator in either of two directions is generally not possible to implement. Active optical systems such as interferometers and switches comprise two optical paths and a phase modulator is used in one of the paths to change the Optical Path Difference (OPD) between the two paths and thus change the optical properties of the system. The phase modulator can be coupled to either optical path for generating a similar effect on the OPD change of the system. Therefore, it is customary to use only one phase modulator in such interferometers or switches.
A second phase modulator can be added into the other optical path of the system. Nevertheless, due to the nature of a phase modulator, each phase modulator generates a similar effect on its associated optical path, i.e. it makes the path length shorter when the driving current is increased and makes the path length longer when the driving current is decreased. As a result, in known systems, the path changes resulting from the phase modulators substantially counteract each other. Therefore, in prior art optical systems where a second phase modulator has been used, it has generally been used as a backup phase modulator in series with the first phase modulator and in the same optical path.
The object of the present invention is to provide an optical system including components defining two or more optical paths for the transmission of light beams and having an improved arrangement for varying the lengths of the optical paths so as to change the phases of light beams transmitted along the optical paths.